Abstract/Description

The fact that there have been no mass invasions of desert or migratory locusts for twenty years or so does not mean that the threat is over. When the rains returned to the Sahelian region of Africa last year there were several localised invasions....

Notes

The fact that there have been no mass invasions of desert or migratory locusts for twenty years or so does not mean that the threat is over. When the rains returned to the Sahelian region of Africa last year there were several localised invasions. We clearly cannot afford to relax our guard. Furthermore, as a result of human activities, other locust species which had never posed a particular threat until recent years are becoming a new a dreaded plague.
For centuries the swarms of locusts which descended on fields and devoured everything in their path represented an unstoppable, inexplicable curse. We can now understand and check these mass invasions thanks to advances in the study of acridids (locusts belong to the family Acrididae) in the development of effective chemical counter measures and in the establishment of supranational agencies to combat locusts.
Our understanding of the periodic appearance and disappearance over time of these plagues of migratory acridids has advanced considerably since the 1930's. Locust populations pass through different phases. In the solitary phase they are not dangerous but, when environmental conditions are particularly favourable, these destructive insects quickly crowd together and enter a gregarious phase. These swarms then move about en masse as the wind and their need for food take them. The spectacular swarms take a terrible toll on crops.
As each individual consumes its own weight in vegetable matter every day, swarms of 50 million locusts devour 100 to 150 tonnes daily.
Preventative action is needed to stop these invasions: locust populations have to be destroyed at an early stage before their behaviour becomes gregarious. It is much harder to take effective action once the swarms have developed. Detailed knowledge of the conditions which encourage these insects to enter the gregarious phase, coupled with constant monitoring of the ecology of high-risk areas, now enables us to predict the likelihood of crowding occuring. Remote-sensing, using NOAA satellite pictures, pinpoints green areas in the desert which often provide the conditions which trigger-off the gregarious phase (concentration of winged individuals and reproduction in situ). Ground-based surveillance and study are still vital for calculating the density of insect populations and monitoring their evolution
Locust control
The main method used today to combat locusts is to kill off locust populations, preferably at the larval stage, with chemicals. In addition to landbased methods (dusting, spreading and spraying by hand or machine) which are easily applied over small areas, aeroplanes and helicopters can be used to halt swarms on the move over large areas and can be used in the desert. Dieldrin, which has a persistent toxic effect on acridids as well as on vegetation, is used only in uninhabited areas. This product is highly effective but very dangerous for people and other animals. Fenitrothion or malathion, which are not as long-lasting but are much less toxic, are preferred in inhabited areas. Al1 chemical products present risks reduced somewhat by the use of very low volume sprayers but they remain an essential weapon in the fight against locusts. Mechanical methods are notoriously unsatisfactory, and no effective methods of biological control have yet been identified in this field.
The fight against locusts has been spectacularly successful. The desert locust, which has a range of 28 million square kilometres from India to the west coast of Africa, has been the subject of an intensive international campaign organized by FAO. In the Sixties, any swarm of locusts which appeared was systematically hunted down. Careful monitoring of the areas in which swarming started and the ecological conditions which encouraged this have kept the pest at bay since 1962. In Africa, this locust breeds in sandy, temporarily wet desert areas: in the flood basins of Saharan wadis and around the Red Sea. Two regional organizations, the Joint Acridid and Bird Control Organization (OCLALAV) in West Africa and the Desert Locust Control Organisation for Eastern Africa (DLCOEA), specially set up to combat this locust, have proved particularly effective. The few incidences of pullulation which occurred in Ethiopia in 1966-68, in the south of Mali in 1978 and in the north of Mali in 1980 were quickly dealt with and had no serious repercussions. Having caused so much devastation over so many years, the desert locust is now well under control
The migratory locust, which breeds in the central delta of the Niger in Mali, in the basin of Lake Chad and in the Blue Nile region of the Sudan also seems to have been hit by the drought which has considerably reduced the amount of land under floodwater which encouraged pullulation. One could be forgiven for thinking that the International Organization to Combat the African Mi-gratory Locust (OICMA) is out of a job.
But although Africa has been free from both locusts for a long time, this does not mean all danger has passed. At any moment climatic and ecological conditions could again encourage these species to start crowding together. Furthermore, irrigation projects sometimes artificially recreate the conditions conducive to locusts. The oases set up by SATEC in the Libyan Desert form an ideal spot for these locusts to reproduce. The insects soon found their way there and started breeding, and stringent measures were required to deal with the problem
In the current lull, the member countries of the organizations set up to combat locusts are becoming increasingly reluctant to pay their contributions. Monitoring is flagging, equipment is no longer maintained or replaced, and money is running out. If quick, mass action were required serious problems could arise. The crop protection authorities, whom governments now prefer to handle the acridid problem, are short of funds and, unlike the locusts, are restricted by national boundaries.
The regional organizations have become victims of the success of the preventive campaigns: the lack of interest they arouse on the part of the countries affected is the major problem facing us today. This was unfortunately brought home to Ethiopia last year when swarms of desert locusts reappeared and devastated several provinces, destroying the long awaited harvests.
New pests
Admittedly, for the last ten years Africa has had to face up to a new scourge. Other grasshoppers, which had not hitherto posed too much of a problem, are now becoming formidable pests. Unlike locusts, their behaviour does not undergo transformation although we now know that they are not entirely sedentary. Although they will not become gregarious, they nevertheless cause concern when their numbers start to grow and when they devour more than 1 to 5 per cent of the harvest (the average level in normal years). They breed rapidly when the environment changes in their favour, particularly since these acridids adapt extremely quickly to new living conditions.
Two species are particularly worrying. Odaleus senegalensis has been proliferating since the major drought of 1969-73. By 1974 this species had spread throughout the Sahel, overrunning 3.5 million hectares (and destroying 350,000 tonnes of cereals). The reappearance of the rains had encouraged eggs to hatch and nymphs to proliferate even more than usual because the drought has reduced predator populations. This grasshopper feeds mainly on millet. At the beginning of the rainy season, the populations feed on young plants in the Northern Sahel. They move southwards at the end of the winter and devour the ears, which are still milky. The increase in millet cultivation has encouraged populations of this pest to expand.
Michel Launois, head of the French Programme of Interdisciplinary Research into the Acridids of the Sahel (PRIFAS), considers the even more destructive Sonocerus variegatus as the 'acridid of the year 2000'. This omnivorous acridid settles in forest clearings in the humid tropics. It moves in when roads are built, electric lines laid or plantations established, wherever it finds the secondary plant cover that it needs. It seems to have a particular predilection for Laos Grass (Chromolaena odorata) which spreads rapidly in cleared areas. The recent drought has helped it expand by decreasing populations of its predators, a fly and a fungus. This pest is no longer a minor problem but has become severe in countries such as the Ivory Coast where it causes enormous damage every year. Furthermore, it has outgrown the forest clearings and has now spread from the Gulf of Guinea towards the Sahel, taking advantage of irrigation projects in Senegal, Mali, Niger and Chad
Over the last few years, these two new pests have caused more damage than desert or migratory locusts. Their proliferation and continual expansion into new areas is worrying. Ploughing up the fields where Zonocerus variegatus lay their eggs and using chemicals to destroy populations which have reached a dangerous density remain the only means of attack. Both methods are expensive and'sometimes difficult to carry out, and provide no more than a temporary answer to the problem.
The long-term solution would be a new form of agricultural development which takes these issues into account. Deforestation and the expansio'n of irrigation to new areas in order to feed the growing population have altered natural ecological balances and transformed harmless insects into chronic pests. The question requires urgent consideration and realistic solutions which will allow development to take place without creating new problems.
Future prospects
Current research in this area is directed at building up a detailed picture of the biology of these acridids, their behavioural patterns and environmentally determined variations. The use of multi-factorial analyses and models may allow us to predict whether the social and biological needs of acridids are, or will, be met by existing or planned projects and will help us to act in a timely and appropriate fashion. The greatest challenge is to find the money to fund research and eradication camoaians.
To summarize, a combination of human activity and drought has served to arrest mass locust invasions, but the lack of water and the impact of man-made projects have caused other acridids to proliferate instead. The return of normal rains to the Sahel last year calls for constant vigilance to prevent swarms of migratory locusts from forming again. Unfortunately, other acridids seem to have settled in and immediate measures are needed now to limit the damage of what may turn out to be the number one scourge facing these tropical developing countries in the year 2000.